1. Field of the Invention
The invention relates to a photomask for use in an exposure step of a semiconductor fabrication process. Also, the invention is concerned with a method of fabricating a semiconductor device, comprising the step of forming resist patterns by executing exposure with the use of the photomask.
2. Description of the Related Art
A photomask is generally obtained by depositing a Cr (chromium) film on predetermined parts of a glass substrate, serving as light shielding parts. Parts of the glass substrate, not covered with the Cr film, serve as light transmitting parts. Exposure light is transmitted through these light transmitting parts or shielded with these light shielding parts, thereby implementing transfer of predetermined mask patterns to a resist or the like as an object for exposure, formed on a semiconductor substrate. After the transfer of the mask patterns to the resist, patterning is executed on the resist, and a semiconductor device is fabricated by use of the resist as patterned.
In the exposure step, the resist is irradiated with light through the photomask after the surface of the semiconductor substrate (also referred to as a wafer at times) is covered with the resist. As a result, a latent image of the mask patterns is formed in the resist. Subsequently, upon the development of the resist, parts of the resist, corresponding to the latent image, are selectively left out or removed, thereby forming resist patterns on the wafer. In the case of pattern fidelity being excellent, the resist patterns are substantially equivalent in shape to the mask patterns. In the exposure step, use is made of a 1× magnification or reduction type projection aligner. In the case of using the reduction-type projection aligner, the size of the respective resist patterns as formed is equivalent to the size of the respective mask patterns, multiplied by a size reduction ratio. In such a case as described, mask pattern designing can be done with ease.
However, along with advance in further microminiaturization of the mask pattern, the influence of an optical proximity effect becomes pronounced, and consequently, the pattern fidelity deteriorates, thereby causing a problem that the resist pattern is found largely deviated from the mask pattern.
Referring to FIG. 13, the optical proximity effect is described simply hereinafter. FIG. 13 is a schematic view showing an exposure system using a typical reduction-type projection aligner. As shown in FIG. 13, the projection aligner comprises an aperture 10, a photomask (referred to at times merely as a mask) 12, and a reduction lens 14. Exposure light emitted from a light source (not shown) is transmitted through the aperture 10, some optical lens between the aperture 10 and the mask 12, the mask 12 and the reduction lens 14 in this order to be irradiated to a wafer 16 with a resist film formed on the surface thereof. The aperture 10 is a light shielding plate having an opening 10a for transmitting light. The mask 12 is a glass substrate 18 with a Cr film 19 deposited at predetermined positions on the surface thereof. When the exposure light passes through the mask 12, there occurs diffraction at light transmitting parts (glass parts), The exposure light after passing through the mask 12 is condensed on the wafer 16 by the agency of the reduction lens 14.
Thus, diffraction occurs at the mask 12. Assuming that respective intervals between line patterns of the mask patterns are designated P, a diffraction angle θ is expressed by expression (1) as follows:P sin θ=nλ  (1)where λ is the wavelength of exposure light, and n is an order. According to the expression (1), the smaller a value P is, that is, the more the pattern is miniaturized, the greater a diffraction angle θ becomes. Because an optical system such as the reduction lens 14 is of a finite size, and the optical system comes to fail arresting light components of a large diffraction angle θ (higher-order light components), light intensity distribution on the wafer 16 deteriorates as compared with information of the mask patterns. As a result, resist patterns formed on the wafer 16 becomes rounder or comes to recede in the extremities thereof. This represents the optical proximity effect.
FIGS. 14, 15, 16, and 17 show an example of the optical proximity effect, respectively. Respective figures (A) thereof show various types of conventional photomasks, and in the respective figures, crosshatched parts correspond to the light shielding parts of the respective photomasks while hollow parts correspond to the light transmitting parts of the respective photomasks. Respective figures (B) thereof show results of simulating resist patterns in the case of exposure being executed by use of the respective photomasks. In the respective figures (B), image patterns together with the resist patterns obtained by the simulation are shown. The respective image patterns are parameters necessary for performing a simulation, representing the shape and size of the respective mask patterns virtually projected on the surface of a semiconductor substrate. More specifically, the size of the mask pattern on the photomask used in the reduction-type projection aligner is reduced down to the size of the image pattern in the respective figures (B) (for example, the size of a pattern on the mask used in a 1:5 reduction-type projection aligner is five times as large as the size of the pattern shown in the respective figures). Further, a region surrounded by a solid line inside respective hollow parts shows the respective resist patterns obtained by performing the simulation (for easy differentiation, an extension line indicating the resist pattern is provided with an arrow). In the respective figures, a X direction and a Y direction, intersecting each other at right angles, are shown, and in the respective figures showing the results of the simulation, the position of the respective image patterns, along the X direction and Y direction, respectively, is shown on a unit of μm.
The resist may be either a positive resist or a negative resist, and in the case of assuming that the resist is the positive resist, the respective resist patterns shown, particularly, in FIGS. 14(B), 15(B), and 16(B), respectively, correspond to a hole pattern. In the case of assuming that the resist is the negative resist, the respective resist patterns shown, particularly, in FIG. 17(B) correspond to a line pattern. Herein, the results of the simulation performed using primarily the positive resist is described.
Simulation conditions are as follows:    (1) illumination condition: Lens NA/σ=0.60/0.75    (2) the wavelength of exposure light: λ=248 nm (KrF laser ray)    (3) resist: positive resist
Lens NA as described above is called a numerical aperture of a reduction lens and is expressed by the following equation (2):Lens NA=n·sin θ2  (2) (n:refractive index)where sin θ2 refers to an angle between the direction of propagation of a light ray after passing through the reduction lens 14 in FIG. 13, and the optical axis of the reduction lens 14.
A resolution power increases when lens NA is increased, and a depth of focus (DOF) increases when lens NA is reduced. For causing lens NA to be changed, it is suffice to change the diameter of a light transmitting part by use of an aperture stop of the lens.
Further, σ as described above is a ratio of the entrance pupil to the exit pupil, that is the diameter Ri of the opening (entrance pupil) 10a of the aperture 10 to the diameter Rl of a lens pupil 14a of the reduction lens 14(exit pupil), and is expressed by the following equation (3):σRi/Rl  (3) 
A pupil corresponds to a scope through which light can pass. The diameter Rl of the lens pupil 14a is proportional to lens NA.
The resolution power, DOF, and a difference in size between an isolated pattern and a nested pattern vary depending on a combination of NA and σ.
In obtaining the results of the simulation, shown in FIG. 14(B) to FIG. 17(B), respectively, use was made of Prolith 2 Ver. 5.07 (tradename) manufactured by FINLE Technologies Company for a simulation tool.
FIG. 14(A) is a schematic illustration showing a conventional photomask 20. The photomask 20 comprises light transmitting parts 22 which are hollow parts, and a light shielding part 24 which is a crosshatched part. In this example, the respective light transmitting parts 22 are designated as a mask pattern. The respective light transmitting parts 22 are in a rectangular shape , and are disposed in a grid format (3 rows×3 columns).
With respective image patterns 26 in a rectangular shape, shown in FIG. 14(B), the length of the longer side (a distance in the X direction) thereof and that of the shorter side (a distance in the Y direction) thereof are in the order of 0.6 μm, and 0.25 μm, respectively. An interval between the rectangular shapes adjacent to each other is in the order of 0.13 μm in the X direction and in the order of 0.18 μm in the Y direction, respectively. Respective resist patterns 28 shown in FIG. 14(B), obtained as a result of the simulation, would be identical in shape and size to the respective image patterns 26 without the influence of the optical proximity effect. In practice, however, the respective resist patterns 28 obtained as a result of the simulation are found rounder or receding as compared with the shape of the respective image patterns 26.
FIG. 15(A) is a schematic illustration showing a conventional photomask 30. The photomask 30 comprises light transmitting parts 32, and a light shielding part 34. The respective light transmitting parts 32 are in the same rectangular shape as shown in FIG. 14(A), but differs in a layout plan. More specifically, the same are disposed in a zigzagged, that is, staggered grid format. Further, an interval between the light transmitting parts 32 linearly adjacent to each other, in the X direction, is equal to the length (in the order of 0.6 μm) of the longer side of the respective light transmitting parts 32. Such a layout of the light transmitting parts 32 is a layout frequently adopted in designing a photomask for fabrication of a semiconductor device.
Respective resist patterns 38 shown in FIG. 15(B), obtained as a result of a simulation, would be identical in shape and size to respective image patterns 36 without the influence of the optical proximity effect. In practice, however, the respective resist patterns 38 obtained as a result of the simulation are found rounder or receding as compared with the shape of the respective image patterns 36.
FIG. 16(A) is a schematic illustration showing a conventional photomask 40. The photomask 40 comprises light transmitting parts 42, and a light shielding part 44. The respective light transmitting parts 42 are in the same rectangular shape as shown in FIG. 14(A), but differs in a layout plan. More specifically, the same are disposed in a zigzagged grid format. Further, an interval between the light transmitting parts 42 linearly adjacent to each other, in the X direction, is shorter than the length (in the order of 0.6 μm) of the longer side of the respective light transmitting parts 42. That is, the light transmitting parts 42 shown in FIG. 16(A) are disposed at a higher density in the X direction in comparison with the layout of the light transmitting parts 32, shown in FIG. 15(A). Such a layout of the light transmitting parts 42 also is a layout frequently adopted in designing a photomask for fabrication of a semiconductor device.
Respective resist patterns 48 shown in FIG. 16(B), obtained as a result of a simulation, would be identical in shape and size to respective image patterns 46 without the influence of the optical proximity effect. In practice, however, the respective resist patterns 48 obtained as a result of the simulation are found rounder or receding as compared with the shape of the respective image patterns 46.
FIG. 17(A) is a schematic illustration showing a conventional photomask 50. The photomask 50 comprises light transmitting parts 52, and a light shielding part 54. As shown in FIG. 17(A), respective light transmitting parts 52 in a line-like shape are disposed so as to be apart from each other in the X direction. The respective light transmitting parts 52 in the line-like shape are called a mask pattern. The length of a shorter side (width) of the respective light transmitting part 52 is in the order of 0.2 μm. An interval between the light transmitting parts 52 adjacent to each other, in the X direction, is in the order of 0.3 μm.
Respective resist patterns 58 as shown in FIG. 17(B), obtained as a result of a simulation, would be identical in shape and size to respective image patterns 56 without the influence of the optical proximity effect. In practice, however, the respective resist patterns 58 obtained as a result of the simulation are found rounder or receding as compared with the shape of the respective image patterns 56.
Thus, as shown in FIGS. 14 to 17, pattern fidelity is found deteriorated due to the optical proximity effect.